Overloading << is part of Stroustrup's iostreams toy and thus enjoyed the advantage that the language's creator was advocating it from the outset (it pre-dates the C++ standard library). I doubt that anything so ugly and dangerous would be standardised even in C++ today.

Because this is actually still a shift operator, it retains all the precedence of the shift operator even when tired humans think of it as as a clever new stream out operator. So if you're in a situation where shift has precedence, the operator fires, and too bad if that wasn't what you'd intended nor what makes sense to a human reading the code.

Oops.

Some languages let you mint new operators (from some limited combinations of symbols) or allow infix function calls, or by some other means provide for you to nicely extend syntax here, but the overload of unrelated operators was IMNSHO an ongoing disaster whose popularity with programmers is unexplained, like C's decision to treat an array as just the pointer to the front of the array.

But I'm a cranky old man, I also don't like the fact that Rust's + concatenates Strings (this isn't inherent in the language since String isn't a built-in type, in Linux kernel Rust nothing like this exists, because Linus would throw a fit, but in the out-of-box runtime environment Strings implement the Add operator as a concatenate operation).